You can delete your existing theme by head over to Appearance -> Themes section. You just need to switch to a default WordPress theme and then you can delete your existing X theme.
Then download the latest version of X theme from your Envato account and then upload the
x.zip file to re-install it. If your Cornerstone plugin is not v3.3.8, you can delete it as well from the plugins section and then head over to X -> Validation section, wait for few seconds to install the latest version automatically.
After the above steps, you should be able to validate your site. In case you're still having issues, you need to follow our manual update guide here (https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62).
Note: In case if you get the same error "X theme folder already exists", you need to connect through FTP and delete your X theme folder manually which is located under (/wp-content/themes/x).
Hope that helps.